1. Field of the Invention
The present invention generally relates to the field of semiconductor wafer testing, and more particularly, to a non-contact method and apparatus for testing such wafers.
2. Discussion of the Related Art
There are a variety of important measurements that must be made on a semiconductor wafer to determine whether it is suitable for further device processing and to make process adjustments. Examples of such measurements include doping concentration measurements, charge time retention measurements, and general leakage measurements. However, present measurement techniques have repeatability problems, and those techniques that utilize metal-oxide-semiconductor (MOS) structures to make the measurements destroy the wafer under test.
Advances in semiconductor technology place new requirements upon electrical characteristic measurement equipment and associated measurement capabilities, for instance, a requirement to monitor epi and ion implant sheet resistance, R.sub.S, in a high R.sub.S range, on the order of greater than 500 ohms/square. Current known measurement apparatus and techniques include probing technology through the use of mechanical probes, such as, well known 4 point probe techniques. Probing technology has limitations, however, due to probing errors.
For instance, in the measuring of sheet resistance of a low dose, shallow implant layer on a semiconductor wafer, the four-point probe technique is highly prone to errors. For instance, use of mechanical probes greatly increases the probability for a probe to poke through an-implant layer. Probes are generally not able to make an adequate ohmic contact to a highly resistive, low dose, implant layer. Furthermore, probe pressures necessary for making ohmic contact with an implant layer can create P-N junction leakage between the implant layer and an underlying opposite conductivity test wafer. For these and other reasons, the mechanical probing techniques are inadequate for the requirements of advanced semiconductor monitoring needs.
It would thus be desirable to provide an alternative method and apparatus for obtaining "4-point probe-like" measurements. Such an apparatus, and method, should be well suited for providing desired "4-point probe-like" measurements and further having an acceptable repeatability and accuracy of measurements suitable for advanced semiconductor monitoring needs.